ExSci Mid Tri Exam (Topic 1&2)

1. Describe the main macronutrients that provide energy for the body

Carbohydrates are the most abundant organic molecules in nature and serve as a primary energy source. The chemical formula for carbohydrates is typically represented as (CH2O)n, highlighting their composition. During digestion and absorption, disaccharides and polysaccharides must be broken down into monosaccharides, which are the simplest form of carbohydrates. Monosaccharides, such as glucose and fructose, do not require further digestion, making them readily available for the body’s energy needs.

When glucose enters the bloodstream, it is transported to the liver, where it is absorbed and packaged for distribution to other tissues. The liver plays a key role in regulating blood glucose levels and ensuring a steady supply of energy. About 5% of liver mass is made up of glycogen, while glucose is stored in muscle tissue primarily as glycogen, comprising approximately 1.5% of muscle mass. Glycogen serves as a rapid energy source during physical activity, with the ability to be converted back into glucose when needed.

Fats, also known as lipids, are another crucial macronutrient, serving multiple functions including energy storage, structural support within cell membranes, and hormone production. Common types of dietary fats include fatty acids, triglycerides, phospholipids, and sterols. Fat molecules, particularly triglycerides, undergo emulsification by bile acids, which increases their solubility in water for better digestion. Pancreatic lipase is the enzyme responsible for breaking down triglycerides into free fatty acids (FAs) and 2-monoacylglycerol. Once digested, absorbed FAs are transported to tissues via lipoproteins—such as chylomicrons, low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Lipids represent the largest energy store in the human body, displaying significant individual variability in distribution and quantity, with women typically having a higher percentage of body fat than men. Fat can be found as subcutaneous fat beneath the skin and visceral fat, which surrounds internal organs, both of which contribute differently to health risks.

Proteins are composed of chains of amino acids, with 20 different amino acids classified as the building blocks of protein. The amino acids are connected by peptide bonds. Essential amino acids, such as histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, cannot be synthesized by the body and must be obtained through the diet. The specific sequence of amino acids in a protein determines its unique three-dimensional structure and function, playing crucial roles in cell signaling, immune response, and muscle contraction.

2. Explain how energy content (caloric value) is quantified.
   Energy conversion for the various macronutrients varies, with fats yielding the highest caloric content. The caloric values are as follows:

• FAT: 37 kJ/g (12 kcal/g)

• Carbohydrates (CHO, mono-): 16 kJ/g (4 kcal/g)

• PROTEIN: 17 kJ/g (4 kcal/g)

• Alcohol: 29 kJ/g (7 kcal/g)
  Understanding these energy values is essential for dietary planning and assessing energy balance.

2. Describe the techniques used to quantify human body composition.

BMI (Body Mass Index) is a statistical measure that assesses body mass relative to height, providing a rough estimate of body fat. However, it is a relatively poor predictor of body fat percentage in certain individuals, particularly those with high muscle mass. It tends to be a reasonable predictor of body fat in weight-stable sedentary populations. The formula for BMI is: BMI = BM (kg) / Ht (m²).

Bioelectrical impedance analysis (BIA) is another method adopted in body composition assessment, operating on an indirect two-compartment model. The principle relies on the fact that electrical conductivity correlates to fat-free mass, with body fat being a poor conductor of electricity. By applying a low-level current (around 50 kHz, 500mA), the analysis estimates body fat percentage (BF%) based on the electrical impedance measured.

The waist-to-hip ratio (WHR) is also an important measure, indicating body fat distribution. A higher WHR suggests visceral obesity, which is associated with increased health risks. This ratio is calculated by dividing the waist circumference (cm) by the hip circumference (cm).

Hydrodensitometry is often termed as the gold standard for body composition analysis. It follows Archimedes' principle, determining body density by measuring the volume of water displaced when a person is immersed. Fat increases buoyancy, and this principle allows for a fairly accurate estimation of body fat, though it is expensive, non-portable, and time-consuming.

Dual Energy X-Ray Absorptiometry (DEXA) employs an indirect three-compartment model and uses low-current x-ray beams delivered at two energy levels to analyze whole-body composition, providing insights into regional and segmental body fat distribution. This method is user-friendly, reliable, and precise but comes with high costs and sensitivity to hydration status.

Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) provide detailed images of body composition through x-ray or magnetic fields, allowing for precise segmentation and differentiation between various tissues. While they deliver high-resolution images, the exposures to radiation (CT) and costs (both MRI and CT) are notable disadvantages.

Skinfold thickness measurements are an indirect assessment technique using calipers to measure subcutaneous fat at various body landmarks. It is a cost-effective, portable method that is heavily reliant on the skill of the technician and proper equipment, with its accuracy affected by hydration status.

4. Compare and contrast the objectivity and validity of various body composition assessment techniques.

5. Explain the importance of body composition and/or dimensions in sport, health, and disease.

In sports, understanding body composition is vital to optimize performance aligned with specific sport demands, prevent injuries, and improve training outcomes. Many athletes need to manage weight classes to maintain competitive edges. In terms of health, body composition offers better indicators of fitness and well-being than weight alone as it affects metabolism, energy levels, and the risk of chronic diseases. Maintaining a healthy body composition promotes healthy aging and mobility. When considering disease, body composition assessments are critical for analyzing risks and tracking progression in conditions such as obesity, diabetes, and cancer. They also help monitor muscle loss, such as in sarcopenia or cachexia, guiding treatment and recovery plans. Overall, body composition provides a richer understanding of an individual’s health status and performance capabilities than weight alone can provide.